Three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy and gel permeation chromatography (GPC) analysis were employed to investigate the distribution and transformation of dissolved organic matter (DOM) in the anoxic-oxic-settling-anaerobic (A + OSA) process for sludge reduction. The A + OSA process showed a good performance in nitrogen and organic pollutants removal efficiency, and reduced sludge yield by 32% under a sludge retention time of 6 h in the anaerobic sludge holding tank (SHT). Parallel factor analysis was used to assess DOM composition from EEM spectra and three fluorescent components were identified: two humic-like components and one protein-like component. In the A + OSA process, the humic-like components were difficult to degraded, while the protein-like component was easily hydrolyzed and adsorbed under anoxic conditions. The fluorescence intensities of the humic-like and protein-like components were both strengthened in the SHT owing to sludge decay under the anaerobic condition. GPC analysis of the A + OSA system showed that the majority of molecules in the influent wastewater with molecular weight (MW) in the range of >250 and 30-50 kDa were mainly transformed into small molecules with MW in the range of 30-250 and <0.5 kDa in the effluent by microorganisms. The DOM in the SHT effluent demonstrated a broader MW distribution and higher intensity than that in the SHT influent, and the percentage of MW between 10 and 30 kDa in the SHT effluent were significantly increased owing to cell lysis and decay in the SHT. (C) 2014 Elsevier B.V. All rights reserved.